In the continuing development and expansion of the communication related fields, there is a continuing need to improve upon the accuracy and rate of information transfer. An often encountered limitation is that various data communication channel networks have already been established. For practical and economic reasons, any improvement in the rate or accuracy of information transfer is distinctly preferred if they can be made while remaining fully compatible with the existing channel network to which they apply.
The nature of the information transferred also substantially constrains the manner that any improvements in rate or accuracy can be obtained. For example, many improvements have been made in the analog encoding schemes utilized for the simple transmission of digital data through a data channel of a network defined bandwidth. Maintaining transfer accuracy requires that the encoding scheme maintain an adequate level of noise immunity depending on the form of encoding used, whether the common frequency shift key (FSK) or any of the phase-shift techniques. These encoding techniques improve the rate of data transfer by increasing the data representation density of the analog waveform, but without significantly increasing the required waveform bandwidth. Thus, greater rates of data can be accurately transferred within the constraining maximum bandwidth of the data channel.
Continuous analog data is another common form of information that is widely communicated through the existing channel networks. An example of "continuous" analog data is, of course, the human voice. Conventional techniques provide quite well for the accurate transfer of such continuous analog data. A distinct problem arises, however, when there is a need to transfer some additional data through the analog data channel at the same time that the continuous analog data is being transferred. Conventional multiplexing of the real time analog data with the additional data immediately incurs two significant disadvantages. The first is that, due to the continuous nature of the analog data, portions are lost each time the additional data is multiplexed into the data channel for transfer. In a practical sense, this loss is acceptable if the multiplex ratio of continuous analog data being multiplexed into the data channel as opposed to the additional data must be such that the loss of real time analog data is minimal. Unfortunately, where the loss of real time analog data is acceptably low, the quantity of additional data that can be multiplexed into the data channel is correspondingly minimized, if not made insignificant. Depending on the particular application in which the real time analog data is being utilized, and the minimum worthwhile rate of additional data being provided through the data channel, either the accuracy of the real time analog data or the rate of the additional data, or both, must be compromised.
The second problem involves the underlying requirement that the demultiplexer at the receiver end of the data channel be closely synchronized with the transmitter end multiplexer in order to accurately separate the transmitted continuous analog and additional data. Therefore, some mechanism is required to initially synchronize and thereafter maintain the synchronization of the respective multiplexer and demultiplexer time bases. One conventional synchronization scheme provides for an initial alignment of the two time bases prior to the beginning of the entire continuous analog data transfer. This method presumes that the continuous analog data naturally occurs as separate messages, each otherwise continuous. This scheme implicitly requires that the transmitter and receiver time bases be of sufficient accuracy, independent of one another, that there is no more than some very minimal amount of drift or relative time difference between the two time bases for the duration of the entire message. Such an "open loop" time base multiplexer/demultiplexer system is typically quite expensive where the messages are long. Artificially shortening the messages by multiplexing in further time base initialization data directly and undesirably impacts the accuracy of the reconstruction of the continuous analog data.
An alternate, conventional synchronization scheme provides for the continuous synchronization of the respective multiplexer and demultiplexer time bases. This type of synchronization scheme is often described as "closed-looped". This alternate scheme superimposes a clock signal onto the analog data at the transmitter for recovery by the receiver. However, a notch filter or the like must be used to erase the clock signal form the analog data. This erasure does not occur without the loss of similar frequency data from the analog data and, therefore, loss of accuracy in the transfer of the analog data.
Thus, there is a need for a communication system that provides for the accurate transfer of continuous analog data in combination with an at least adequate rate of additional data, all transferred within a typically narrow band, communication network data channel.